Effect of Acute Increases in Retrograde Shear on Near‐Infrared Spectroscopy Derived Microvascular Reactivity

Abstract

Periods of high vascular resistance lead to increases in retrograde shear rate, this augmented shear rate has been demonstrated to acutely decrease macrovascular endothelium‐dependent flow‐mediated dilation. However, it is unknown if similar exposures to a period of retrograde shear rate negatively impacts the microvascular response via near‐infrared spectroscopy (NIRS).PurposeDetermine the effects of acute retrograde shear rate on microvascular responses in healthy, young humans. It was hypothesized that exposure to increased levels of retrograde shear rates would elicit a decrease in post‐occlusive microvascular reactivity measured via non‐invasive NIRS.Methods17 participants (22.7 ± 2.91 yrs) completed the study. Microvascular function was measured as the post‐occlusive reactive responses for tissue saturation index (TSI%) and total hemoglobin + myoglobin ([Hb] total) in the m. rectus femoris before and after a 30‐minute retrograde shear rate treatment. The retrograde shear rate in the treatment leg (T) was achieved via a cuff placed below the knee inflated to 75 mmHg, meanwhile the contralateral leg served as control (C). Shear rate was measured in the superficial femoral artery. Due to the limitations of non‐invasive 2D Doppler ultrasound interrogating smaller vessels, the evaluation of changes in shear rate in the microcirculation could not be performed.ResultsThe cuff inflation significantly increased the retrograde shear in the treatment leg (71.2 ± 46.9 s−1 vs. 156.5 ± 63.5 s‐1, P<0.01) compared to the control leg (74.8 ± 58.6 s−1 vs. 66.6 ± 45.7 s−1, P=0.37). The peak post‐occlusive response for TSI% (T: 79.9 ±10.2 %; C: 76.8 ± 11.9%, P=0.15) and [Hb] total (T: 44.7± 29.7 μM C: 51.5 ± 43.3 μM, P=0.06) were not different between legs following the intervention. Similarly, the rate of change for StO2 (T: 6.0 ± 7.0 % s−1 104; C: 5.0 ± 6.0 % s−1 104) and [Hb] total (T: 4.0 ± 4.0 μM s−1 104; C: 3.0 ± 4.0 0 μM s−1 104) were not different between legs.ConclusionIncreases in retrograde shear rate achieved via application of cuff pressure does not alter post‐occlusive NIRS derived indices of microvascular reactivity. Results may be due to different retrograde shear patterns elicited along the arterial tree, limitations of NIRS, or limited effect of retrograde shear on microvascular function.